Method of repairing defective pixels

ABSTRACT

Methods are disclosed by which the effects of a defective electromechanical pixel 20 having a beam 30 and a hinge 32,34 are mitigated. These methods may damage the hinge 32,34 or the beam 30 and comprise the step of applying a voltage sufficient to damage the hinge 32,34 or beam 30 of said electromechanical pixel 20 by mechanical overstress, thermal overstress, electrochemical reaction, or thermally induced chemical reaction. Other methods are also disclosed.

This application is a continuation-in-part of Ser. No. 08/148,753, filedNov. 5, 1993 now U.S. Pat. No. 5,387,924, which is a continuation ofSer. No. 07/965,835, filed Oct. 23, 1992, now U.S. Pat. No. 5,289,172.

CROSS-REFERENCE TO RELATED PATENTS

The following coassigned patent applications are hereby incorporatedherein by reference:

    ______________________________________    Pat. No.  Filing Date       TI Case No.    ______________________________________    5,096,279 Nov. 26, 1990     TI-14481A    5,083,857 Jun. 29, 1990     TI-14568    ______________________________________

FIELD OF THE INVENTION

This invention relates to digital micro-mirror devices (DMD's), alsoknown as deformable mirror devices, and more particularly to a method ofmitigating the effects of defective pixels in such devices.

BACKGROUND OF THE INVENTION

DMD's have found numerous applications in the areas of opticalinformation processing, projection displays, and electrostatic printing.See references cited in L. Hornbeck, 128 X 128 Deformable Mirror Device,30 IEEE Tran Elec. Dev. 539 (1983).

A great number of the applications described in Hornbeck, supra, useDMD's operated in a bistable mode as described in U.S. Pat. No.5,096,279, incorporated by reference herein. The details of '279 will besummarized in some detail herein, but briefly in the bistable mode of aDMD a deflectable beam or mirror may be deflected to one of two landingangles, ±θ_(L), by underlying electrodes to which an address voltage isapplied. At either landing angle (±θ_(L)) an extremity of thedeflectable mirror lies in contact with an underlying device substrate.Generally, in one orientation, the deflectable mirror is "on", bright,or in other words reflecting light in the field of view. In the otherorientation, the mirror is "off", dark, or not reflecting light in thefield of view.

It has been discovered in prior art DMDs that a possible manufacturingdefect is that of stuck mirrors. In such an instance, individual mirrorsmay not change between "on" and "off" states in response to changes inaddress and bias voltages. As such, the mirrors are always "off" oralways "on". Always "on" defects or bright defects are particularlynoticeable and objectionable. The defective pixels stick for a number ofreasons including but not limited to: a defective addressing elementunderlying the mirror, a poor electrical connection between the mirrorand a reference voltage, or a surface defect at the point of mirrorcontact.

SUMMARY OF THE INVENTION

The present invention is the first to recognize that the defectivepixels may have characteristics that allow the effects of their defectsto be mitigated. That is, the invention presents a means of changing apixel tipped in one direction permanently to a pixel that is completelymissing, rendered non-reflective, partially reflective, or becomesnon-deflected in either direction. In accordance with an embodiment ofthe present invention, the circuitry used for addressing and biasingmirrors may be employed to effect permanent changes upon the defectivepixels without destroying non-defective pixels. Essentially, a highvoltage is applied to the mirror contact relative to electrodes whichlie on or within the device substrate. The defective mirrors are thendrawn forcefully down to substrate through electrostatic attraction. Theforces are applied to either break the mirror hinges, or make a directcontact to electrodes on the DMD substrate resulting in high currentswhich may destroy the hinges or beam.

Although it is possible to provide addressing circuitry to apply theseforces to individual mirrors, selectively, this requires knowledge ofthe location of the defective pixels. Accordingly, another embodiment ofthis invention applies forces to the entire body of mirrors in such amanner as to effectively destroy only those mirrors which are defective.This is accomplished using the fact that properly functioning pixels canbe reset to a flat state, in which the plane of the pixel beam isparallel to the plane of the device substrate, while the stuck onescannot. It is known that the mechanical structure of the pixel cannotrespond instantaneously to the applied voltage. In fact air resistancehas been shown to limit the movement of the pixels. Because the stuckpixels are already tipped they will contact their correspondingelectrodes before the functional pixels will. This time lag betweencontact of stuck pixels to the electrodes and contact of non-defectivepixels to their electrodes can be used to destroy the hinges and/orbeams of the defective pixels. Hence, an embodiment of the currentinvention applies short duration, high amplitude electrical pulses tothe entire body of pixels whereby the defective pixels will beselectively destroyed.

In yet another embodiment of this invention, defective pixels mayactually be repaired and not destroyed by application of a current to anoverlying electrical superstructure, which comprises the pixels,relative to an underlying superstructure, which comprises the addresselectrodes. According to Fundamentals of Physics, p. 560 (Haliday &Resnik eds., 2d edition, 1981), "it is a well-established principle thattwo long parallel wires carrying currents exert [attraction] forces oneach other." Conversely, two wires carrying "antiparallel currents . . .repel each other." If currents or eddy currents could be induced intothe pixels in relation to the underlying address electrodes, pixelswould attempt to pivot in order to maximize the distance between themand the underlying address electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1a-c illustrate in perspective, cross sectional elevation, andplan views, a functional rendering of a preferred embodiment pixel;

FIG. 2 illustrates deflection of a mirror of the preferred embodiments;

FIG. 3 illustrates defect mitigating voltages which may be applied in afirst preferred embodiment in which the voltages are selectively appliedto defective pixels;

FIG. 4 illustrates defect mitigating voltages which may be applied in asecond preferred embodiment in which the voltages are applied to theentire body of pixels;

FIGS. 5a-c schematically illustrate use of the preferred embodiment DMDfor electrophotographic printing;

FIG. 6a illustrates a top view of a partial array of preferredembodiment mirrors;

FIG. 6b illustrates a top view of a preferred embodiment mirror showingmajor hidden features;

FIG. 6c illustrates a detailed cross sectional view as indicated in FIG.6b of a preferred embodiment mirror; and

FIGS. 7a-d illustrate, in partial cross section, progressive formationof a mirror of the preferred embodiment.

Corresponding numerals and symbols in the different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1a-c illustrate in perspective, cross sectional elevation, andplan views a functional rendering of a preferred embodiment mirror. Asillustrated by these figures, pixel 20 is operated by applying a voltagebetween beam 30 and electrodes 42 or 46 on substrate 22. Beam 30 and theelectrodes form the two plates of an air gap capacitor and the oppositecharges induced on the two plates by the applied voltage exertelectrostatic force attracting beam 30 to substrate 22, whereaselectrodes 40 and 41 are held at the same voltage as beam 30. Theelectrostatic force between electrodes 42,46 and beam 30 causes beam 30to twist at hinges 34 and 36 and be deflected towards substrate 22.

FIG. 2 is a schematic view of the deflection of beam 30 with anindication of the charges concentrated at the regions of smallest gapfor a positive voltage applied to electrode 42. For typical CMOSvoltages, for instance 5 volts, the deflection is in the range of 2degrees. Of course, if hinge 34 were made longer or thinner or narrower,the deflection would increase as the compliance of hinge 34 varieslinearly with the inverse of its width and directly with the square ofits length and inversely with the cube of its thickness. For a DMDoperating in its bistable mode, the beam design is such that the beam's30 deflection is defined by the landing angles, ±θ_(L), at which pointthe beam 30 contacts the DMD substrate on landing electrodes 40,41. Notethat the thickness of beam 30 prevents significant warping of beam 30due to surface stress generated during processing, but that the thinnessof hinge 34 allows for large compliance. FIG. 2 also indicates thereflection of light from deflected beam 30 as may occur during operationof the DMD.

FIG. 3 illustrates the defect mitigating voltages applied in the firstpreferred embodiment of the present invention. This embodiment might useaddressing circuitry as disclosed in U.S. Pat. No. 5,096,279,incorporated by reference herein, which discloses the addressing andbiasing scheme of a typical bistable DMD in great detail. To use thistype of addressing and biasing circuitry to mitigate defects requiresminor modification obvious to one skilled in the art of circuit design.By using this addressing circuitry, once the defect locations aredetermined each defective pixel selectively addressed and pulses will beapplied to damage the beams and/or hinges for defect mitigation. Inparticular, the pulses applied for mitigation of defects will ofnecessity be greater in amplitude, A₁ and/or duration, D₁, than thoseused for addressing the micromirrors 30 of the DMD. Theamplitude/duration of the first embodiment mitigating voltages must besufficient to destroy the hinges 34,36 of the pixel 20. The defectmitigating circuitry might serve a dual purpose as the addressingcircuitry, or the circuitry may be parallel to the circuitry normallyused for addressing the mirrors 30 as in the prior art.

In a second preferred embodiment, the mitigating pulses as shown in FIG.4 might be applied to the entire body of pixels simultaneously, orperhaps to subsets of the entire body of pixels. In such instance thepulses would be of short duration and high amplitudes. Theamplitude/duration, A₂ /D₂, should be sufficient to destroy the hinges34,36 of most defective pixels 20 without damaging an unacceptablenumber of functional pixels 20 which would be normally undeflectedbecause of the short duration of the pulses. Preferably, the pulseswould be periodic with a period τ.

If the vertical axes of FIGS. 3 and 4 were made to represent thedifferential currents applied to the pixels 20 relative to theunderlying address electrodes 42, the method of actually correctingdefective pixels 20 might be effected. The pulses of current resultingin an application of FIG. 4 might be used advantageously to avoiddamaging pixels 20 from prolonged repellant forces from a continuousapplication of such a current differential between the overlyingsuperstructure of pixels and the underlying superstructure of addresselectrodes.

The differential currents applied to the pixels 20 to correct defectivepixels 20 might be applied in a number of ways. To have the salutaryeffect of repelling defective pixels 20 from the address electrodes 42to which they are stuck, the opposing currents need only be transitory.So, for example, the currents could be applied through blockingcapacitors or by motion of external magnets inducing eddy currents inthe circuitry.

To summarize the addressing circuitry as disclosed in U.S. Pat. No.5,096,279, incorporated by reference herein, a bistable pixel 20 can bemade addressable by establishing a preferred direction for rotation. Ifboth address electrodes 42 and 46 are grounded, then small perturbationswill cause beam 30 to randomly rotate and collapse to one of the landingelectrodes 40,41 upon application of the differential bias V_(B) to beam30 and landing electrodes 40 and 41. However, if prior to application ofthe differential bias V_(B), address electrode 46 is set to a potentialthen a net torque will be produced to rotate beam 30 towards landingelectrode 41. Symmetrically, applying the triggering potential toaddress electrode 42 will rotate beam 30 to landing electrode 40 uponapplication of the differential bias V_(B).

In accordance with either embodiment disclosed hereinabove, if thedifferential bias V_(B) is applied to beam 30 relative to landingelectrodes 40,41, with a great enough amplitude the beam 30 may not onlycollapse, but the hinges 34,36 will actually be permanently damaged by alarge mechanical overstress such that the beam may lie non-deflectedupon the underlying substrate in neither an "on" or "off" state.Sufficient deflection may actually cause the beam 30 to contact one ofaddress electrodes 42,46 such that a high current flows through the beam30 and hinges 34,36. Heating from this high current may be utilized totemperature overstress, melt, or destroy through thermally inducedchemical reaction, the hinges 34,36. Sufficient current may even damageby melting, or render non-reflective by a thermally induced chemicalreaction, the beam 30 in addition to the damage caused to hinges 34,36.This rendering of the beam 30 to a non-reflective state may be adesirable condition relative to its bright, "on" state. Any thermallyinduced chemical reaction may take place in a gas as is well known inthe art. Any temperature stress by current conduction may be inducedwithin a gas or vacuum. A higher pressure or more viscous gas having agreater damping effect than would air at one atmosphere of pressure maybe used to increase the damping on the non-defective beams 30, therebyincreasing the time advantage due to the previously mentioned lagbetween application of a voltage to a non-defective beam 30 and contactof that beam 30 with the DMD substrate (contrasted with the defectivebeam 30 already being in contact with the DMD substrate). This extratime advantage can be used to more effectively chemically react orstress the defective beam 30.

Many thermally induced chemical reactions can be envisioned which couldbe employed in this invention. These reactions include but are notlimited to electrically-induced thermal oxidation andelectrically-induced vapor anodization. For example, the defectsresulting from a poor electrical contact between the beam 30 and thebias circuitry are mitigated. In this embodiment, an electrochemicalreaction could be performed in a reactive vapor selective to thedefective pixels by applying a voltage to all beams 30 and utilizing thefact that only beams 30 with a proper contact will be affected by thiselectrochemical reaction. This electrochemical reaction could beutilized to mask the non-defective beams 30 from subsequent depositionof a non-reflective or partially reflective surface on the defectivebeams 30. Alternatively, the non-defective beams 30 may be masked fromsubsequent chemically reactive vapor processes to etch or otherwisedamage the defective beams 30 or the hinges 34,36.

A linear array 310 of preferred embodiment pixels 20 could be used forelectrophotographic printing as illustrated schematically in FIGS. 5a-c.FIG. 5a is a perspective view and FIGS. 5b-c are elevation and planviews showing system 350 which includes light source and optics 352,array 310, imaging lens 354 and photoconductive drum 356. The light fromsource 352 is in the form of a sheet 358 and illuminates linear array310. Light from the areas between pixels 20 forms sheet 360 which is thespecularly reflected sheet of light. The light reflected from negativelydeflected beams form sheet 361. The light reflected from positivelydeflected beams 30 pass through imaging lens 354 within sheet 362 andfocus on drum 356 within line 364 as a series of dots, one for eachdeflected beam 30. Thus a page of text or a frame of graphicsinformation which has been digitized and is in raster-scanned format canbe printed by feeding the information a line at a time to array 310 toform dots a line 364 at a time on drum 356 as drum 356 rotates. Thesedot images are transferred to paper by standard techniques such asxerography. If 0 is the deflection angle of beam 30 when on landingelectrodes 41, then sheet 362 is normal to linear array 310 when theangle of incidence of sheet 358 is 20° from the normal to linear array310. This geometry is illustrated in FIG. 5b and permits imaging lens354 to be oriented normal to linear array 310. Each positively deflectedbeam produces an image 355 of light source 352 on imaging lens 354 asschematically shown in FIG. 5c for three beams.

FIGS. 6a-c illustrate a top view, a top view showing major hiddenfeatures, and a detailed cross section of a partial array of preferredembodiment mirrors. This preferred embodiment structure uses amulti-level deformable mirror structure and method of manufacturing asdisclosed by Hornbeck in U.S. Pat. No. 5,083,857. As shown in FIG. 6a,this structure provides a greatly improved area of rotatable reflectivesurface for a given pixel size. The underlying hinges, address andlanding electrodes are shown as dotted lines in FIG. 6b. Beam supportpost 201 rigidly connects beam 200 to underlying torsion hinge 401.Details of the underlying hinge and electrodes are shown in FIG. 6b.Beam support post 201 allows beam 200 to rotate under control of hinges401 which in turn are connected to posts 406. This allows rotatablesurface (beam) 200 to rotate under control of an electrode supported byposts 403. Beam 200 lands in contact with landing electrode 405. Contact402 extends through the substrate and is in contact with the underlyingaddress electronics. The construction and operation of this device willbe discussed hereinafter. FIG. 6c illustrates beam 200 rotation 200a tolanding angle -θ_(L) and rotation 200b to landing angle +θ_(L). Alsoshown are address electrodes 404 which control the movement (200a, 200b)and landing electrodes 405 positioned at the other end of the see-sawswing of beam 200. The manner of controlling the rotational movement ofbeam 200 is detailed in U.S. Pat. No. 5,096,279 filed on Nov. 26, 1990.

The process sequence for the hidden hinge architecture is shown in FIGS.7a-7d and consists of five layers (hinge spacer, hinge, electrode, beamspacer, and beam). Referring now specifically to FIG. 7a, the processbegins with a completed address circuit 503 including contact openingsformed in protective oxide 501 of the address circuit. The addresscircuit is typically a two metal layer/poly CMOS process. The contactopenings allow access to the second level metal (METL2) 502 bond padsand to the METL2 address circuit output nodes.

Still referring to FIG. 7a, hinge spacer 701 is spin-deposited over theaddress circuit and patterned with holes 702 that will form the hingesupport posts and electrode support posts and contacts. This spacer istypically 0.5 μm thick and is a positive photoresistant deep UV hardenedto a temperature of 200° C. to prevent flow and bubbling duringsubsequent processing steps.

As shown in FIG. 7b, the next two layers 703 and 704 are formed by theso-called buried hinge process. An aluminum alloy that forms the hingeis sputter-deposited onto the hinge spacer. This alloy is typically 750Å thick and consists of 0.2% Ti, 1% Si and the remainder Al. A maskingoxide is plasma-deposited and patterned in the shape of hinges 401. Thishinge oxide is then buried by a second aluminum alloy layer 704 that isto form the electrode (typically 3000 Å thick).

With further reference to FIG. 7b, a masking oxide is plasma-depositedand patterned in the shape of the electrodes 404, the electrode supportposts 406 and the beam contact metal 405. Next, a single plasma aluminumetch is used to pattern the hinges, electrodes, support posts and beamcontact metal. The electrode metal overlying the hinge region is etchedaway, exposing the buried-hinge oxide which acts as an etch stop. Whenthe plasma aluminum etch is complete, regions of thin hinge metal 703and thick electrode metal 704 have been simultaneously patterned. Themasking oxide is then removed by a plasma etch.

Next as shown in FIG. 7c, beam spacer 705 is spin-deposited over thehinges and electrodes and patterned with holes that will form beamsupport posts 201. Spacer 705 determines the torsion beam angulardeflection and is typically 1.5 microns thick and is a positivephotoresistant. It is deep UV hardened to a temperature of 180° C. toprevent flow and bubbling during subsequent processing steps. Note thatno degradation of hinge spacer 701 occurs during this bake, because thehinge spacer was hardened to a higher temperature (200° C.). Next, analuminum alloy that is to form beam 200 (typically 4000 Angstroms thick)is sputter-deposited onto beam spacer 705. Next, masking oxide 707 isplasma-deposited and patterned in the shape of the beams. The beam isthen plasma etched to form the beams and beam support posts. Thiscompletes the process at the wafer level. Masking oxide 707 on beam 200is left in place. The wafers are then coated with PMMA, sawed into chiparrays and pulse spin-cleaned with chlorobenzene. Finally, the chips areplaced in a plasma etching chamber, where masking oxide 707 is removedand both spacer layers 701 and 705 are completely removed to form theair gaps under the hinges and beams as shown in FIG. 7d.

While the preferred embodiment has been described in terms of damaging abright, defective pixel, the difference between a permanently brightpixel and a permanently dark pixel is merely one of orientation. It isthus inherent in this invention that the characteristics of dark pixelsmay also be affected by the methods described herein. Although thedescribed embodiment of this invention is the damaging of defectivepixels, the invention also comprehends restoring defective pixels totheir proper function.

Although this description describes the invention with reference to theabove specified embodiments, the claims and not this description limitthe scope of the invention. Various modifications of the disclosedembodiment, as well as alternative embodiments of the invention, willbecome apparent to persons skilled in the art upon reference to theabove description. Therefore, the appended claims will cover suchmodifications that fall within the true scope of the invention.

A few preferred embodiments have been described in detail hereinabove.It is to be understood that the scope of the invention also comprehendsembodiments different from those described, yet within the scope of theclaims. Words of inclusion are to be interpreted as nonexhaustive inconsidering the scope of the invention. Implementation is contemplatedin discrete components or fully integrated circuits in silicon, galliumarsenide, or other electronic materials families, as well as inoptical-based or other technology-based forms and embodiments.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method of mitigating the effects of a defectiveelectromechanical pixel, the method comprising the step of applyingelectromagnetic energy to said pixel to render said pixel operable. 2.The method of claim 1 wherein said electromagnetic energy is provided bya differential current applied to said pixel relative to an underlyingstructure.
 3. The method of claim 2 whereto said differential current isan eddy current induced in said pixel relative to said underlyingstructure.
 4. The method of claim 2 wherein said underlying structure isa set of address electrodes.
 5. The method of claim 2 wherein saiddifferential current is applied to said pixel through blockingcapacitors.
 6. The method of claim 1 wherein said electromagnetic energyis provided by the motion of external magnets.
 7. The method of claim 6wherein said motion of said external magnets induces eddy currents insaid pixel relative to an underlying structure.
 8. The method of claim 1wherein said electromagnetic energy is provided as a set of periodicpulses of electromagnetic energy applied to said pixels.
 9. The methodof claim 1 wherein said pixel is an electromechanical pixel having ahinge.
 10. A method of mitigating the effects of a defectiveelectromechanical pixel, the method comprising the step of applying adifferential current to said pixel relative to underlying addresselectrodes, said differential current causing said pixel and saidaddress electrodes to repel each other whereby said defective pixel isrendered operable by said repulsion.
 11. The method of claim 10 whereinsaid electromagnetic energy is provided as a set of periodic pulses ofelectromagnetic energy applied to said pixels.
 12. The method of claim10 wherein said electromechanical pixel has a hinge whereby saidrepulsion operates to separate said pixel from said address electrodes.13. The method of claim 12 wherein said repulsion operates to maximizesaid separation between said pixel and said address electrodes.